DE-OS 3 419 969 discloses a spring-controlled tensioning device that may be adjusted during operation. This device consists essentially of a journal bearing which is fixed during operation and, attached thereto, a hub moving at an angle with an eccentric tension roller and a spring located between the carrying units and the hub. The spring causes the tension roller to be preloaded against a drive belt and automatically compensates for any changes in tension of the drive belt during operation. The carrying unit runs on bearings on a pin and can be adjusted via a lever through a movable angle, by means of an adjusting screw.
This system allows adjustment of the predetermined tension. In order to obtain as uniform a tension as possible in mass production, the carrier is provided with an indicator operating in conjunction with a scale on the hub. Accordingly, a predetermined angle of rotation is set by the adjusting screw, with reference to a zero setting. This method provides a uniform tension force in mass production only if springs having relatively small tolerances are used and if no changes in spring characteristics are expected during final mounting, or as a result of other components that could affect tolerances. This cannot be avoided in mass production, however, so that the pre-determined tension forces achieved with the prior devices, and according to the above described adjustment method, are widely distributed. The prior devices are subject to great variations in belt drive mechanisms insofar as interference and temperature compensation during operation are concerned, in particular with regard to wear and tear.